The present invention relates to a sensor element, in particular a sensor element for analyzing gas mixtures, for example gas mixtures in an internal combustion engine, and a method for its calibration.
In sensor elements operating by the diffusion limit current principle, the diffusion limit current is measured at a constant voltage applied to both electrodes of the sensor element. For an exhaust gas produced in a combustion process, this diffusion limit current depends on the oxygen concentration as long as the gas diffusion to the xe2x80x9cpump electrodexe2x80x9d determines the rate of the reaction taking place. Known sensors of this type operating by the polarographic measuring principle are designed so that both the anode and the cathode are exposed to the gas to be measured; the cathode has a diffusion barrier to ensure operation in the diffusion limit current range. The known limit current sensors are normally used to determine the xcex value of gas mixtures, which denotes the ratio of total oxygen to the amount of oxygen needed for full combustion of the fuel, for example, of an air-fuel mixture to be burned in one cylinder; the sensor displays the oxygen level of the exhaust gas using limit current measurement with a pump voltage in a predefined range.
Thanks to simplified and inexpensive manufacturing methods, it has been proven advantageous in the past few years to industrially manufacture sensor elements made of ceramic foils using screen-printing techniques. Planar sensor elements can be more simply and inexpensively manufactured from plate- or foil-shaped oxygen-conducting solid electrolytes made, for example, of stabilized zirconium dioxide; such sensor elements are coated on both sides with an internal and an external pump electrode and with the corresponding printed circuit. The internal pump electrode is advantageously positioned in the edge area of a diffusion channel, through which the measured gas is passed, and which serves as a gas diffusion resistance.
From German Patent No. 35 43 759, European Patent No. 0 142 993, and European Patent No. 0 194 082 B1, other sensor elements and detectors are known having plate- or foil-shaped, oxygen-conducting solid electrolytes, with electrodes arranged thereon, as well as a common diffusion channel as common features.
A shortcoming with the above-described sensor elements has been that the sensor element made of plate- or foil-shaped elements contains a printed diffusion barrier, whose layer thickness is subject to natural process fluctuations because of the manufacturing technology used, in particular, final sintering of the diffusion barrier. This produces undesirable fluctuations in the pump current. To date, it has not been possible to manufacture-larger amounts of sensor elements with diffusion barriers having constant characteristics in a satisfactory and cost-effective manner, because the pump current fluctuated with each lot and had to be adjusted in a costly manner.
The advantage of the sensor element according to the invention is that the diffusion resistance of the present diffusion barrier can be linearly adjusted by selectively modifying the diameter of the gas intake orifice. The diffusion resistance can be selectively adjusted to the requirements in a simple manner by the use of a gas intake orifice produced in a subsequent operation.
Calibration during the manufacturing process of the sensor element is advantageously used for calibrating a sensor element, so that calibration can be performed in a simple manner on the blank prior to final sintering of the sensor element. Thus calibration takes place almost simultaneously with the process steps, so that the finished product does not have to be reworked in an expensive and time-consuming manner.
In a preferred embodiment, the diameter of the gas intake orifice is varied either mechanically or by laser drilling; in the latter case drilling in a particularly simple and elegant manner is made possible even after sintering.
The pump current of a sensor element for a previously defined diameter of the gas intake orifice at a previously selected pump voltage is advantageously measured. Then the measured pump current is correlated with the diameter of the intake orifice and the optimum pump current. This is possible because in a first approximation the following equation applies:
dB≈IP,
where dB is the diameter of the gas intake orifice and Ip is the pump current. This offers the possibility of more precisely adjusting the sensor element by varying the orifice diameter in order to set the target values of the pump current which relates to dB.
Preferably, at least one sensor is selected from a lot of identical unsintered sensor elements without gas intake orifice. This allows the entire lot of hundreds or thousands of unsintered sensor elements to be calibrated prior to final sintering by using a single sensor element.
Advantageously, a gas intake orifice with a specified diameter, approximately corresponding to a pump current of 4.8 mA, is produced in this selected sensor element. Then the selected sensor element with the specified diameter of the gas intake orifice produced is sintered, so that the theoretical final characteristics of the entire lot of sensor elements can thus be characterized.
In a particularly advantageous embodiment, the pump current of this selected sintered sensor element is measured at a preselected pump voltage, preferably 1000 mV. Thus the value of the measured pump current can be adjusted to the target value and the diameter of the gas intake orifice using the above-mentioned approximation. The relationship between the measured pump current and the desired target value is obtained using a simple ratio, so that the optimum diameter of the gas intake orifice of the entire lot of sensor elements can be determined using a simple mathematical procedure.
In a preferred process step, the gas intake orifice with the optimized diameter is now produced in the lot of remaining unsintered sensor elements, so that the entire lot of unsintered sensor elements can be adjusted to the optimum pump current in this simple procedure.